Are You Pulling My Leg?
Content Warning: Self-Mutilation
Betty sits at a table with her hands placed on top of it. As far as an observer can tell, there’s nothing physically wrong with her hands; she has one right and one left, both connected to her arms and body. However, she says that her left hand is not there—it has disappeared. In fact, she can see the table that is underneath where her arm should be . How can this be if her left hand is resting on the table, visible to everyone else? What Betty is experiencing is called asomatognosia, a condition in which a person loses body ownership of a limb. The sense of ownership of one’s body is not a guarantee. In fact, ownership of a limb can be lost, altered, or gained.
The woman, wearing a purple coat, walks her dog in Central Park. A runner runs behind her. The woman sees in a bubble on her head that her left limbs disappear into specks and the runner walks her dog for her.
Fundamentally, body ownership is an awareness of your own body, in which you feel like your body belongs to yourself . It’s your perception of and connection to your own body, where the external and internal sensations you experience feel personal to you . Body ownership is continuous and constant—you experience it during voluntary movement, passive action, and even when stationary . However, body ownership is not absolute; it can easily be altered or manipulated, even if a person is able-bodied and neurotypical . For example, in the Rubber Hand Illusion, a participant can be momentarily tricked into believing that a fake rubber hand is their own. This experiment involves placing a participant’s real hand and a fake rubber hand side by side on a table. The real hand is hidden behind a screen, while the fake rubber hand is visible in front of them. Then, an experimenter strokes both the hidden real hand and visible fake hand at the same time, causing the participant to connect what they feel in their real hand to what they see in the fake hand. Subsequently, the participant briefly adopts the fake hand as their own . Ultimately, the experiment proves a crucial aspect of body ownership: it can be modified.
Where’s my leg?
Stemming from the Greek phrase “without body knowledge,” asomatognosia is a condition in which a patient loses a sense of agency over their limb . For instance, patient ASG, hereafter referred to as Anne, was a highly educated 53-year-old woman who had undergone surgery to remove a brain tumor . A few months after the surgery, while walking her dog, she suddenly felt that the limbs on the left side of her body did not exist. It was as if they were completely gone. Though Anne was still able to see her left hand and leg, she sensorially felt that they were no longer there. Consequently, she was often left wondering how she could continue to walk her dog with her left hand, despite it being absent . Anne’s case was one of sensory asomatognosia. Sensory asomatognosia is when a person doesn’t feel their limb; in contrast, visual asomatognosia is when a person doesn’t see their limb. In the example mentioned earlier, Betty was experiencing visual asomatognosia because she could not see her arm . Research also reports that some patients with asomatognosia could not identify their own hand while it was placed on a table next to a researcher’s hand . Regardless, in all cases of asomatognosia, the patient lacks some perceptual capability surrounding their own limb.
People often develop asomatognosia after suffering from damage to the right hemisphere of the brain . Since the right hemisphere deals with sensations and information coming from the left side of the body, patients usually experience asomatognosia on the left side of their body . This relationship between the right hemisphere of the brain and the left side of the body is important for the conditions regarding body ownership mentioned later in this article. While patients with asomatognosia tend to have damage to various parts of the right hemisphere, including areas such as the medial frontal region, one particularly relevant area that patients typically incur damage to is the temporo-parietal section of the brain . The parietal lobe deals with awareness of the position of the body in space, while the temporal lobe is involved with auditory and some visual processing . Most importantly, however, is that the junction between the temporal lobe and parietal lobe is often associated with the ability to orient oneself and control attention . Thus, damage to these areas may result in a patient losing their ability to localize and focus on their limb.
A person's left limbs and chest (in the hue of purple) overlaps
with a person's right limbs and chest (in hue of yellow).
That’s not my leg, it’s hers!
Stemming from the Greek phrase for “body delusions,” somatoparaphrenia is a subtype of asomatognosia that also involves the loss of body ownership of a limb, most commonly on the left side of the body . However, patients with somatoparaphrenia
believe that their limb belongs to someone else—usually, a person who is in the room with them . During one case study, a patient looked at his own left hand, then insisted that his hand belonged to the researcher who was in the room with him. After the researcher expressed that the hand in question was the patient’s own hand, the patient did not believe it, exclaiming, “This is your hand!” Even when the patient was actively and voluntarily moving his hand, he still believed that the hand belonged to the researcher, saying, “Yes I told you, I can see it well, this is your hand! My hand is resting down” .
Consistent with asomatognosia, patients with somatoparaphrenia also experience damage to temporo-parietal regions of the right hemisphere . However, unlike asomatognosia, somatoparaphrenia is associated with damage to white matter, which is composed of axons —the wires of neurons. Axons are responsible for sending information throughout the brain, so damage to white matter may mean that the brain has a harder time sending out information . While white matter damage is spread throughout various parts of the right hemisphere, one relevant area that is commonly damaged is the thalamus, a region responsible for receiving sensory information from various parts of our body [7, 10]. Lesion to the thalamus might mean that a patient has a harder time receiving information from certain parts of the body, for example, the sense of touch coming from the left hand. While the mechanism by which patients lose awareness of their limbs is more understood, there is a lack of research regarding the neuroanatomical correlation as to why a patient attributes their limb to someone else.
That’s not my leg, please cut it off!
While asomatognosia involves a patient not being able to see or feel their limb, and somatoparaphrenia involves a patient attributing their limb and its sensations to someone else, body integrity identity disorder (BIID) is a condition in which a person denies ownership of one or more of their limbs and usually desires amputation or paralysis of the limb(s) . The affected limb is usually healthy and fully functional . Patients with BIID are fully capable of feeling sensations of the limb; in fact, they often become hyper-fixated on it, reporting exaggerated pain . People with BIID have a discrepancy between how their body actually looks and their mental representation of their bodies . So, for example, despite physically having four functional limbs, a patient’s mental representation of their body might exclude their left leg . To simulate their desired body—one without the limb they do not recognize as their own—some people with BIID use crutches or wheelchairs . Furthermore, a case study of a small group of patients reported that subjects with BIID often felt “complete” once the limb had been removed .
Unlike asomatognosia and somatoparaphrenia, BIID is not caused by lesions to the brain but instead is an identity disorder that most patients testify to having their whole life . In one study, the average age that patients started consciously experiencing issues with body ownership was seven years old, with some patients experiencing issues as early as three years old . People with BIID often have a smaller right superior parietal lobule [11, 14]. The superior parietal lobule, located in the upper region of the parietal lobe, is linked to one’s awareness of the position of their body parts and overall mental representation of their body . So an atrophic right superior parietal lobule might mean a lowered awareness of one’s body parts. Additionally, patients with BIID often have reduced grey matter concentration . Unlike white matter, which transmits information, grey matter is composed of the bodies of neurons that gather and process information . This reduction of grey matter usually occurs in the right paracentral lobule of the parietal lobe, which deals with the motor and sensory functions of the lower limbs of the body [11, 16]. So, a decrease in grey matter in the right paracentral lobule might have damaging effects on how the brain gathers and processes information about the legs. Altogether, the brains of patients with BIID generally may have greater difficulty with retaining awareness and integrating information pertaining to their limbs.
A person (in the hue of purple) places their amputated right calf into a hole in their brain. The hole is caused by the shrinking of their right superior parietal lobe.
That’s my leg, that’s my other leg, and that’s my other other leg!
Unlike the three previously mentioned conditions involving a rejection of body ownership, supernumerary phantom limb is a condition in which a patient believes that they have more limbs than they actually do . Despite only having two arms and two legs, one patient, E.P., hereafter referred to as Emma, felt that she had three arms and three legs, with the extra limbs appearing on the left side of her body . She started to experience her extra limbs roughly three weeks after receiving surgery for a ruptured aneurysm. To explain her condition, Emma described to researchers an accident that occurred while she went shopping. She had all of her purchases in two bags, one in her right hand and one in her left; however, she felt as if she had a third arm, also carrying a bag. Since Emma knew she only bought two bags worth of stuff, she was worried that the bag in her third arm was accidentally taken from another shopper. Notably, when Emma’s left arm was repeatedly touched or when she visually confirmed that she had two arms, Emma did not perceive herself to have an extra limb .
Like the previously described types of asomatognosia, supernumerary phantom limb is usually experienced after sustaining damage to the right hemisphere of the brain . Specifically, the condition correlates with suppressed activity in the secondary somatosensory cortex, as determined by neuromagnetic recordings . The secondary somatosensory cortex is involved with the consolidation and organization of information, such as sensory and motor information, from both sides of the body . So, suppressed activity in the secondary somatosensory cortex may result in the brain improperly synthesizing sensorimotor information from both halves of the body. This may lead the brain to assume that the body has more limbs than it actually does .
These disorders concerning body ownership often have large impacts on patients’ ways of life. In Anne’s case, her perception of the left side of her body disappearing led to her experiencing balance issues and difficulty walking . Patients with somatoparaphrenia often feel frustrated, and sometimes accuse others of stealing their limb . Those affected by BIID often experience dysphoria, emotional duress, and disruption of their social and personal lives . Furthermore, these conditions can be extremely confusing for a patient to comprehend, such as in Emma’s case . Altogether, these conditions often negatively affect the way patients experience the world, so developing treatments is imperative.
There are a variety of treatments that can help improve a patient’s sense of body ownership. Asomatognosia and somatoparaphrenia are so similar that a treatment known as Caloric Vestibular Stimulation (CVS) works for both conditions [22, 23]. CVS involves pouring cold water into a patient’s ear to change the temperature within the vestibular system. The vestibular system coordinates with the sensory processing region of the brain to help maintain balance and preserve the mental representation of the body . The change in temperature causes the patient’s eyes to repetitively shift towards the ear that the water was poured into. This movement of the eyes produces the sensation that the body is moving and increases tactile awareness in the affected limbs . Unfortunately, this treatment is temporary, usually lasting for only a couple of minutes .
Studies have shown that Virtual Visual Feedback (VVF) is another effective treatment for supernumerary phantom limb . VVF involves a patient looking at their head and neck in a full-length mirror, with a sheet covering the view of their lower body. A video of a different person’s lower body is then projected onto the sheet. The patient is instructed to imagine moving their lower body in the same way as the projected body. Researchers hypothesize that this method helps with the reintegration of sensorimotor information. Research suggests that prolonged use of VVF, such as a 12-week treatment plan, can reduce symptoms of supernumerary phantom limb for up to 12 weeks post-treatment .
While CVS and VVF show promising signs of helping patients who have issues with body ownership, treatments for BIID are much more controversial. Currently, one treatment for BIID is to amputate the patient’s unwanted limb . In rare cases, some surgeons have agreed to perform an elective amputation of a patient’s healthy limb [26, 27]. However, if doctors do not agree to amputate the limb, patients often turn to riskier solutions by performing self-mutilation on their unrecognized limbs, which can result in life-threatening injuries . If a patient inflicts irreparable damage to the limb, surgeons have little choice but to perform a proper medical amputation . In a small case study, patients reported not feeling symptoms of BIID after amputation . However, in an alternate study, some patients' desire for amputation of a different limb came back even after getting one of their limbs amputated, suggesting that amputation may not be an effective long-term solution to BIID [29, 31]. Other treatments, such as psychotherapy or pharmaceutical drugs, have varying effects on BIID patients but usually do not permanently cure their symptoms [12, 29]. In the end, some doctors believe that it is ethical to perform an elective amputation to prevent patients from further injuring themselves . However, others believe that elective amputation of a healthy limb violates doctors’ rule of “do no harm” since it involves purposefully giving a patient a disability and exposing patients to the general risks of having major surgery .
The illustration depicts the imagination of a person (in the hue of purple). The person carries eight shopping bags with their six arms. The person's secondary somatosensory cortex in the right hemisphere is shaded.
In an age where Photoshop, plastic surgery, and filters distort appearances, perhaps it is more common than one would think for a person to feel disconnected from their body. Studying conditions such as asomatognosia, somatoparaphrenia, BIID, and supernumerary phantom limb may aid us in understanding how psychological and neuroanatomical aspects of body ownership work together to create a whole, or sometimes disjunct, comprehension of our own bodies . By comparing the affected brain regions of each condition, we may be able to establish explanations for concrete neural correlates of body ownership; for example, many of these conditions deal with the right hemisphere [5, 9, 11, 14, 21]. However, the relationship between specific brain regions and their corresponding aspect of body ownership, such as why damage to parts of the right hemisphere result in patients with somatoparaphrenia attributing their limb to someone else, still needs more research to be fully understood [11, 29] . Also, through studying these rare conditions, scientists might be able to understand more about more common disorders of body ownership, such as Phantom Limb.
Due to the rarity of these conditions, disorders like BIID are critically under-researched, which results in many patients going undiagnosed and untreated . Consequently, patients continue to live with the mental, emotional, and lifestyle impacts of these conditions. Future research on these disorders should seek to establish better diagnostic criteria and treatment options .
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